Behind the scenes at major art museums, conservators are hard at work, keeping masterpieces looking their best. Their methods are meticulous — and sometimes surprising.

The painting conservation studio at the National Gallery of Art in Washington, D.C., is filled with priceless works sitting on row after row of tall wooden easels, or lying on big, white-topped worktables.

The studio is where I first met Senior Conservator Ann Hoenigswald years ago as she was fixing the sky on one of Claude Monet's impressions of the Rouen Cathedral in France. Bits of paint had flaked off over time, and Hoenigswald was carefully mixing her blue to match the old master's. Seeing the painting outside of its fancy frame, it felt like being inside the artist's studio. (I greatly wanted to try my hand at filling in some tiny bare spot in Monet's sky, which had once been covered by paint. Of course, the thoroughly professional Hoenigswald politely refused to hand over her brush.)

Conservators must take classes in studio art, art history and chemistry. Sometimes guidance comes from artists themselves. For example, Vincent van Gogh wrote to his brother Theo, asking for specific shades of paint — Prussian Blue, Ultramarine, Geranium Lake. Painters in earlier centuries rarely left such clues.

Conservator Michael Swicklik peers through thick lenses that give him a 3-D view of a 15th-century canvas that Italian Renaissance artist Fra Angelico may have painted. They can't be sure, because The Entombment of Christ is in awful condition — freckled with pocks where paint flecked off, and the gold on the saint's halo has worn away. The entire surface is dulled from varnish that's aged to the color of caramels.

Swicklik gets to work with cotton, solvent mixture and a bamboo stick. (Sometimes they just use spit, which gets grime off nicely!) He focuses in on the caramel-colored varnish obscuring the saint's robe — moving over the surface in very light circles, to avoid abrading the paint.

Varnish is the enemy here. Painters, dealers or buyers often put a clear coat of it on to preserve a painting, or give it a nice sheen. Jay Krueger, Head of Painting Conservation, says over time the varnish ages and actually changes the colors of the painting.

"You remember that sky being blue and it's kind of green now, or, you'd remember that this was a lovely silvery dress and it's yellow now," he says. "It's just a matter of that surface, that transparent layer, discoloring over time. So it'll turn reds more orange, it'll turn blues kind of greenish. It darkens the light colors and, in an odd way, it flattens out and lightens the dark colors."

There are all sorts of chemicals involved in the quest to remove the offending varnish, so big blue vacuum tubes — they look like elephant trunks — hang from the ceiling, sucking up fumes and smells.

"You don't want a 40-year career cut short because you're in a room full of open solvents," Krueger says.

The solvents are tailored to meet the needs of a particular painting. Conservator Joanna Dunn is wearing blue rubber gloves to protect her hands from the strong solvent she's working with. Looking through a very fancy microscope, she bends over a 16th-century Tintoretto called Summer. The big canvas — it's more than 3'x6' — shows a zaftig blonde, reclining in a field, her right breast peeking out from her pretty pink drape. For some reason, a parrot turns his back on her. Armed with cotton swab, skinny stick, solvent and a scalpel, Dunn goes after the usual suspect: Varnish.

"This coating is so old I can't dissolve it without harming the paint," Dunn explains. "So the way to do it is to soften it with the chemicals that I'm using, and then ... it becomes gelatinous and I can push it off with the scalpel."

She does this all verrrrrrry carefully. At some point she'll put down the scalpel, pick up a paintbrush and fill in any spots that are missing paint.

"I'm only going to put my inpainting in the area where the paint is missing," she says. "I'm not going to cover any of the original paint."

In addition to varnish, conservators also need to get rid of paint that was applied in earlier restorations and then replace it with colors that match sometimes centuries-old originals. They hope to leave these canvases in better shape so that future conservators have an easier time of it when their turn comes.

Every day, these conservators hop between centuries and styles to preserve masterpieces for future art-lovers. A few years ago, Hoenigswald had a 19th-century Mary Cassatt on one easel, and a 16th-century El Greco on another — and they almost seemed to be in conversation with each other.

Mary Cassatt was a great admirer of El Greco's work, Hoenigswald explains: "I was practically in tears thinking: Oh my god, if she ever thought she'd literally be side by side ... it was a very emotional."

Moments like these, she says, can make these behind-the-scenes conservation studios feel downright magical.

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RACHEL MARTIN, HOST:

That painting you see on the wall of an art museum is the product of the artist and the times in which he or she lived. It is also the result of today, no matter how old it is, a major museum's conservator's work to keep the art looking its best. Their methods are meticulous and sometimes surprising.

As part of our summer series we're calling Backstage Pass, NPR special correspondent Susan Stamberg visits the conservation lab at the National Gallery of Art here in Washington, D.C.

SUSAN STAMBERG, BYLINE: Go in a main entrance. Walk to a door that opens with a passkey and down a long hallway. It looks like a school that you might have attended in 1904 - beige fake tile walls, darker beige linoleum floor. You're on the ground floor, but it feels like the basement.

It doesn't look like a museum. I'm going into painting conservation G115. We don't have to show you our passports? Or...

STAMBERG: Well, it's for security. The conservation room is filled with priceless paintings sitting on row after row of tall wooden easels or lying on big, white-top work tables. I head down the long room to senior conservator Ann Hoenigswald. I'd met her years ago on my very first trip to this backstage lab. Then, she was working on one of Monet's series of impressions of the Rouen - that's R-O-U-E-N - Cathedral in France, filling in tiny dots of sky that had flaked off over time.

And I watched and watched. It was like being in the artist's studio because it was so available. There it was, no frame, no fancy lighting. And I said to you, oh, please, could I just paint in one little blue dot? And you said...

ANN HOENIGSWALD: No (laughter).

STAMBERG: She had to protect the Monet - also her reputation and her job. She knew I'd tell. Ann had to mix her blue to match Monet's. Conservators must take classes in studio art, art history and chemistry for their repair work. Sometimes guidance comes from artists themselves.

HOENIGSWALD: Vincent Van Gogh writes back to his brother, Theo, and says, will you send me Prussian blue and ultramarine and three large tubes of lead white and things like that?

STAMBERG: Conservator Michael Swicklik peers through thick lenses that give him a 3-D view of a 15th-century canvas the Italian Fra Angelico may have painted. They can't be sure because "The Entombment Of Christ" is in awful condition, freckled with pocks where paint flecked off, gold on the saint's halos worn away, dulled all over from varnish that has aged to the color of caramels. Swicklik gets to work with cotton and a stick.

SWICKLIK: So we roll a little swab on a bamboo skewer kind of thing and then...

STAMBERG: Yeah. And then you dipped it in water.

SWICKLIK: No. This is actually a solvent mix...

STAMBERG: Oh, that's the solvent.

SWICKLIK: ...A solvent mixture. Yeah.

STAMBERG: Sometimes they just spit - gets grime off nicely.

You're going after the caramel on the saint's robe.

SWICKLIK: Yup.

STAMBERG: And you're gently touching it with the solvent...

SWICKLIK: Very lightly.

STAMBERG: ...In circles, light circles.

SWICKLIK: A lot of time, just a rolling action. So you - what you want to do is minimize any chance of abrading the surface as you go.

STAMBERG: Varnish is the enemy here. Painters or dealers or buyers put a clear coat of it on to preserve a painting or give it a nice sheen. Jay Krueger, the head of painting conservation, says, over time, the varnish ages and actually changes the colors of the painting.

JAY KRUEGER: You know, you remember that sky being blue, and it's kind of green now. Or you'd remember, you know, this is a lovely, silvery dress, you know? And it's yellow now. It's just a matter of that surface, that transparent layer, discoloring over time. So it'll turn reds more orange. It'll turn blues kind of greenish, darkens the light colors. And in an odd way, it kind of flattens out and lightens the dark colors.

STAMBERG: Conservators work to restore the artist's original vision, so they remove the darkened varnish.

Not so fast. First, they switch on one of the seven big blue tubes. They look like elephant's trunks suspended from the ceiling. They are vacuums curling kind of creepily down over various workspaces. They suck up fumes and smells and get rid of them. Whoosh.

KRUEGER: You know, you don't want to have a 40-year career cut short because you're in a room full of open solvents.

STAMBERG: Conservators tailor the solvents to meet the needs of a particular painting. Joanna Dunn is dealing with solvents so strong she needs blue rubber gloves to use them. Looking through a very fancy microscope, she bends over a 16th century Tintoretto, "Allegory Of Summer." The big canvas - it's more than 3-by-6-feet big - shows a zaftig blonde reclining in a field, her right breast just peeking out from her pretty pink drape. For some reason, a parrot turns his back on her. Armed with cotton swabs, skinny stick and solvent, Joanna Dunn goes after the usual suspect - varnish.

JOANNA DUNN: This coating is so old, I can't dissolve it without harming the paint. So the way to do it is to soften it with the chemicals that I'm using. And then I'm going back with a scalpel. And it actually sort of becomes gelatinous. And I can kind of push it off with the scalpel.

STAMBERG: Very carefully. At some point, she will put down the scalpel, pick up a paintbrush and fill in any spots that are missing paint.

DUNN: I'm only going to put my inpainting in the area where the paint is missing. I'm not going to cover any of the original paint.

STAMBERG: Conservators also need to get rid of paint that was applied in earlier restorations and then replace it with colors that match sometimes centuries-old originals and do it so that future conservators have an easier time when their turn comes. So I guess Ann Hoenigswald was right not to let me play Monet that day. But these professional conservators get to do it every day, going from one century, one style to another for future art lovers.

A few years ago, Ann had a Mary Cassatt - 19th-century American - on one easel and an El Greco - 16th century, Spain. El Greco was on another easel. The juxtaposition made Ann philosophical and something else.

HOENIGSWALD: Mary Cassatt admired none of the old masters the way she did El Greco. And I was practically in tears thinking, oh, my God. If she ever thought she'd be, literally, side-by-side - it was very emotional, really. That's what happens in a conservation studio.